The present invention relates to a control circuit for a tone generator and in particular to a control circuit that is adapted to cause the tone generator to produce a plurality of perceptively distinguishable audible sounds, including a chime sound, a pulse tone and a steady tone.
Current federal regulations require that all automobiles be equipped with devices that will provide a four to eight second audible warning whenever the automobile is attempted to be operated without the seat belts properly fastened. Such devices typically take the form of a buzzer unit that is controlled by a bimetallic timer circuit which is adjusted to time out within the prescribed time period. In addition, the same buzzer unit is typically utilized to also provide an audible signal to indicate other monitored conditions, such as when the keys are left in the ignition or when the headlamps are left on after the ignition is turned off.
Devices of this type have two basic disadvantages. Not only is the sound produced by such buzzer units unpleasant and often irritating, but the same audible sound is produced for all three warning conditions. Hence, it is often not immediately discernable by the operator of the vehicle which of the monitored conditions is triggering the alarm.
Thus, it is one of the principal objects of the present invention to provide an improved control circuit that is adapted to cause the tone generator to which it is connected to produce readily distinguishable audible signals for each of the conditions monitored. In particular, the present control circuit is adapted to drive a tone generator to produce a chime sound, a pulse tone, or a steady tone, depending upon the particular enabling signal received. Thus, it can readily be seen that the present invention is particularly suited for use in an automobile to control an audible warning device that is utilized to monitor the three conditions noted, namely, the seat belts, the headlamps, and the ignition key. Moreover, it will also be appreciated from the following description of the preferred embodiment herein that the present invention is adapted to drive the tone generator so that the sounds produced are pleasant to the ear.
In addition, it will also be seen that the control circuit of the present invention is reliable, relatively simple in design, and therefore exceptionally low in cost. Moreover, the size of the circuit board required for the control circuit is small, thus making the control circuit further suitable for automotive use where space is at a premium.
In general, the control circuit for the present invention comprises three oscillator circuits: a high frequency oscillator, a low frequency oscillator, and a middle frequency oscillator. The oscillators comprise simple logic gate circuits that are adapted to produce square wave output signals. The high frequency oscillator and the low frequency oscillator are utilized to produce the chime sound. In particular, the high frequency oscillator establishes the tone of the chime and the low frequency oscillator provides the "striking" rate of the chime. As will subsequently be described in greater detail, the high frequency oscillator signal is pulse width modulated in a novel manner to produce the desired decay effect in the amplitude of the sound.
The steady tone is produced simply by driving the tone generator with the output signal from the high frequency oscillator. The pulse tone, on the other hand, is generated by enabling the middle frequency oscillator and utilizing the output signal therefrom to control the enabling of the high frequency oscillator. Thus, the tone produced for the steady tone is pulsed at the rate of the output signal from the middle frequency oscillator.